1. Technical Field
The present invention relates to stators for electric rotating machines that are used in, for example, motor vehicles as electric motors and electric generators, and to methods of manufacturing the stators.
2. Description of Related Art
Conventionally, there are known stators for electric rotating machines which include a hollow cylindrical stator core, a stator coil, and an outer cylinder.
The stator core is comprised of a plurality of stator core segments that are arranged in the circumferential direction of the stator core to adjoin one another in the circumferential direction. Further, to reduce iron loss of the stator, each of the stator core segments is formed by laminating a plurality of magnetic steel sheets in the axial direction of the stator core. Moreover, the stator core has a plurality of slots that are formed in the radially inner surface of the stator core so as to be spaced from one another in the circumferential direction of the stator core. The stator coil is mounted on the stator core so as to be received in the slots of the stator core. The outer cylinder is fitted on the radially outer surfaces of the stator core segments so as to fasten them together.
Moreover, there is also known a method of shrink-fitting the outer cylinder on the radially outer surfaces of the stator core segments. More specifically, according to the method, the inner diameter of the outer cylinder is set to be less than the outer diameter of the stator core. In the shrink-fitting process, the outer cylinder is first heated, thereby causing the inner diameter of the outer cylinder to become greater than the outer diameter of the stator core. Then, the outer cylinder is fitted onto the radially outer surfaces of the stator core segments all of which together make up the radially outer surface of the stator core. Thereafter, the outer cylinder is cooled at room temperature until the difference in temperature between the outer cylinder and the stator core segments becomes zero. As a result, the stator core segments are fixed together by the fastening force of the outer cylinder
However, with the above method, the fastening force of the outer cylinder is applied radially inward to the stator core segments, inducing compressive stress in the stator core segments in the circumferential direction of the stator core. The compressive stress may deteriorate the magnetic characteristics of the stator core, thereby lowering the performance of the electric rotating machine.
Japanese Patent Application Publication No. 2007-189786 discloses a stator in which the stator core segments are each made of a powder compact (or green compact) so as to reduce the iron loss of the stator. Moreover, to fix the stator core segments which are received in the outer cylinder, there are formed a lower clamping portion at the lower end (i.e., one axial end) of the outer cylinder and a plurality of upper clamping portions at the upper end (i.e., the other axial end) of the outer cylinder. The upper clamping portion and the lower clamping portions together clamp the stator core segments in the axial direction of the stator core, thereby fixing the stator core segments. More specifically, the lower clamping portion is formed as an annular flange that protrudes radially inward from the radially inner surface of the outer cylinder. The lower clamping portion has the stator core segments resting thereon, so that the lower end faces of back core portions of the stator core segments are in pressed-contact with the lower clamping portion. On the other hand, the upper clamping portions are equally spaced in the circumferential direction of the outer cylinder. Each of the upper clamping portions is first raised up from the upper end of the outer cylinder and then bent downward to have a tip part thereof in pressed-contact with the upper end face of the back core portion of a corresponding one of the stator core segments.
However, with the above configuration, in operation of the electric rotating machine, magnetic flux flows also through the upper and lower clamping portions of the outer cylinder that are respectively in pressed-contact with the upper and lower end faces of the back core portions of the stator core segments, thereby increasing the iron loss of the stator.
Japanese Patent No. 4562093 discloses a stator in which the outer cylinder is fastened to the radially outer surface of the stator core by a protrusion-recess engagement. Specifically, as shown in FIG. 27, the stator core 30A has a recess 35A formed in the radially outer surface thereof, while the outer cylinder 50A has a protrusion 52A that protrudes radially inward from the radially inner surface of the outer cylinder 50A. The protrusion 52A of the outer cylinder 50A is pressed into the recess 35A of the stator core 30A by a punch 61A that is arranged radially outside of the outer cylinder 50A. Consequently, the protrusion 52A of the outer cylinder 50A is brought into contact with both the circumferential side walls of the recess 35A (i.e., the pair of interior walls of the stator core 30A which face each other in the circumferential direction of the stator core 30A with the recess 35A formed therebetween). As a result, the outer cylinder 50A and the stator core 30A are fastened together, so that they cannot rotate relative to each other.
However, with the above configuration, a large pressing load is required for the punch 61A to press the protrusion 52A of the outer cylinder 50A into the recess 35A of the stator core 30A. Consequently, in some cases, it may be difficult to secure a sufficient contact area between the protrusion 52A and the circumferential side walls of the recess 35A. As a result, without a sufficient contact area between the protrusion 52A and the circumferential side walls of the recess 35A, local stress concentration may occur in the protrusion 52A, thereby causing cracks and/or a fatigue fracture to occur in the protrusion 52A.